Devices of this description generally comprise a disk or platter, rotatable about a vertical axis, on whose upper surface the granular material to be sifted is continuously deposited. The particles coming to rest on the disk are rotatably entrained and are radially accelerated by the centrifugal force, thus leaving the disk periphery with a velocity which should be sufficient to carry the heavier particles across a rising air stream while the lighter ones are swept up and directed into a separate channel. Such a device is know, for example, from U.S. Pat. No. 2,913,109.
According to other prior proposals (see, for example, German Pat. No. 1,456,798) the centrifuging platter or slinger disk is spacedly overlain by a cover rotating jointly therewith, the particulate material being ejected through a narrow annular gap formed between the disk and the cover.
For effective separation by an air sifter surrounding the disk, it is desirable that all particles exiting from the gap initially follow substantially the same trajectory regardless of size. With particle sizes spread over a wide range, e.g. from a few microns to several hundred microns, this goal is not easily attained since the frictional contact between the particles and the disk surface increases with particle size so that the larger and heavier particles are more readily entrained, and therefore accelerated, than the others. This is especially true where, for more intense acceleration with a given energy input, the upper disk surface is of upwardly concave frustoconical shape so that the contact between that surface and the particles is enhanced by the centrifugal force acting thereon.
Slinger disks have also been provided in the past with a set of upstanding radial webs for more positive rotative entrainment of the deposited particles. The presence of these webs, however, tends to bunch the ejected particles in a number of peripherally separated streams, particularly if the webs extend all the way to the disk periphery, with resulting lower efficiency of the associated air sifter. If the webs terminate short of the disk periphery, marked differences in acceleration are again observed.